The recent trend towards the miniaturization of electronic circuits is driven by consumer demand for smaller and light-weight electronic devices such as cellular phones and portable computers. Often, the heaviest component in an electronic device is the battery. Smaller batteries are able to provide less power. As batteries become smaller, integrated circuits (ICs) need lower working voltages and power consumption to prevent the battery from discharging too rapidly.
An amplifier is a linear electronic circuit that may be used to amplify an input signal and provide an output signal that is a magnified replica of the input signal. Amplifiers are used in a variety of electronic circuit design applications. As appliances and circuit designs continue to decrease in size and increase in speed, the need for low power, low noise, current efficient amplifier circuitry increases.
Amplifiers have various performance requirements depending on the function they are used for in a circuit. A microphone bias amplifier should have the following attributes: 1) low noise, 2) high power supply rejection ratio (PSRR); 3) low quiescent current, or rather, low overall current usage; 4) the ability to drive high current levels with an output voltage as close to the power supply rail as possible to obtain a good acoustical gain; 5) low output impedance for rejection of any coupled noise; and 6) the use of as little silicon area as possible.